I don't think arithmetic can be done outside the variable commands, and the dollar sign should be replaced with "v_" in the variable command, I think. Try doing this:
variable L equal 500
variable Ldiv2 equal v_L/2
variable L3div2 equal v_L+v_Ldiv2
region 1 block -\{L3div2\} INF INF INF INF INF The dollar sign \() in the fourth line is intentional. It's been awhile since I've used variables, though, and IIRC, the syntax has relatively recently changed. (IIRC, it used to be that one needed to do "div(x,y)" not "x/y".)